Betatron electromagnet

ABSTRACT

A betatron electromagnet which comprises a magnetic circuit with pole pieces restricting an operating zone for mounting a betatron vacuum chamber and each having a solid central part and at least three ribs protruding from the central part but not exceeding the limits of the operating zone, and a field winding.

United States Patent [191 Chakhlov et al.

BETATRON ELECTROMAGNET Filed: June 8, 1972 Appl. No.: 260,736

US. Cl 335/210, 317/123, 328/237 Int. Cl. H01f 7/06 Field of Search 335/210, 213; 328/233,

References Cited UNITED STATES PATENTS Verster et a1. 335/210 Apr. 16, 1974 2,558,597 6/1951 Westendorp 328/237 2,942,106 6/1960 Bennett 328/237 X 3,175,131 3/1965 Burleigh et al. 335/210 Primary Examiner-James D. Trammell Assistant Examinerl-larry E. Moose, Jr. Attorney, Agent, or Firm-Eric H. Waters ABSTRACT A betatron electromagnet which comprises a magnetic circuit with pole pieces restricting an operating zone for mounting a betatron vacuum chamber and each having a solid central part and at least three ribs protruding from the central part but not exceeding the limits of the operating zone, and a field winding.

6 Claims, Drawing Figures BETATRON ELECTROMAGNET The invention relates to induction electron accelerators or betatrons used in physical and biological research as well as in flaw detection and medicine, and, more specifically, to betatron electromagnets.

Known in the art are betatron electromagnets, comprising a magnetic circuit with pole-pieces restricting an operating zone for mounting a betatron vacuum chamber, and a field winding with the pole pieces shaped as conic discs and with a flat central part.

In the betatron electromagnets so designed, due to a non-uniform distribution of the magnetic flux in the airgap zone the pole-piece steel is not fully loaded, since in the flat central part of a pole piece magnetic induction reaches the saturation level (about 15,000 gauss) whereas in the vicinity of the equilibrium orbit with the betatron ratio (2:1) maintained magnetic induction is about 3,000 to 5,000 gauss.

A considerable magnetic leakage, practically unused, from the end faces of the pole pieces reduces the efficiency of the installation and increases its dimensions and weight.

In addition, the focusing forces affecting the accelerated electron beam, are relatively weak for the field gradient n is within n 1.

In such designs the cooling of pole pieces involves difficulties.

The object of the invention is to provide a betatron electromagnet with pole pieces so designed as to form a controlling magnetic field with improved focusing properties, to reduce the leakage field from the polepiece end faces and to improve the pole-piece cooling conditions.

This-object is accomplished in a betatron electromagnet compri'sing a magnetic circuit with pole pieces restricting an operating zone for mounting a betatron vacuum chamber, and a filed winding, wherein, according to the invention, each pole piece of the electric magnet has a solid central part and at least three ribs protruding from the central part and not exceeding the limits of the operating zone.

It is advisable to place the ribs of one pole piece opposite the ribs of the other.

It is advisable that the rib surfaces facing the operating zone be made concave so that the maximumdistance points of these surfaces relative to the betatron median plane should be placed near the cylindrical surface the guide path of which is an averaged equilibrium orbit.

The ribs of one pole-piece may be turned in their plane relative to the ribs of the other.

The embodiment of the betatron electromagnet according to the present invention makes it possible to form in betatrons a time-varying controlling field with a field gradient, variable in magnitude and azimuthly alternating, which is known to possess the best focusing properties.

A decrease of the pole-piece and faces makes it possible to reduce approximately by 40 to 50 percent the leakage flux from these surfaces and, at the same time, to increase the cooling surface as well as to improve heat-removal conditions.

The overall reduction of the magnetic circuit weight may amount to 20 percent.

The invention will now be described by way of examples with reference to the accompanying drawings in which:

FIG. 1 illustrates a sectional view of a betatron electromagnet, according to the invention;

FIG. 2 is a section taken along line IIlI of FIG. 1, according to the invention;

FIG. 3 illustrates one of the embodiments of the electromagnet pole-piece ribs, according to the invention;

FIG. 4 illustates the lay-out of the electromagnet pole-pieces, according to the invention.

FIGS. 1 and 2 illustrate two projections of one of the embodiments of the present electromagnet with six radial ribs 1 protruding from cylindrical central parts 2 of poles 3 and 4.

The betatron electromagnet is split in a median plane 5 and comprises two identical halves 6 and 7. Both halves 6 and 7 are braced with pins (not shown in FIGS.

1 and 2), one of which passes through a center hole 8. The magnetic flux is produced by windings 9 mounted around the end faces 10 of the ribs 1. A reverse magnetic circuit 11 is used to close the magnetic flux. The betatron ratio (2:1), which holds good for betatrons I with the present pole-piece design, is achieved by placing central inserts 12 separated by non-ferromagnetic insulation spacers 13, between the pole pieces 3 and 4. The ribs 1 belonging to one of the pole pieces 3 or 4 are so designed that the distance between side surfaces 14 of the ribs 1 increases, as the radius is increased. The ribs 1 belonging to the pole-piece 3 are placed opposite theribs, belonging to pole piece 4. The surfaces 15 of the ribs 1 facing the operating zone 16 and used for mounting the betatron vacuum chamber may be so designed that an air-gap 17 formed by the surfaces 15 may increase radially, remain unchanged or decrease radially.

In the embodiment described hereintofore the surfaces 15 of the ribs 1 of the pole piece 3 run parallel to the surfaces 15 of the ribs 1 of the pole piece 4 while the air gap 17 remains unchanged radially. The length of the ribs 1 is so selected that an averaged equilibrium orbit l8 divides them approximately into halves.

The ribs 1 are built up from electrical steel laminations 19 which may be set along the ribs, across or at some angle to axes 20 of the ribs 1. The latter is preferable for it provides the best phase structure of the controlling field formed by said ribs in the operating zone 16.

FIG. 3 illustrates another embodiment of the betatron electromagnet wherein, as distinct from the embodiment shown in FIG. 1 and 2, the surfaces 15 of the ribs 1 on the pole pieces 3 and 4, facing the operating zone 16, are made concave while the maximum distance points 21 of the surfaces 15 relative to the betatron median plane 5 are placed near the cylindrical surface, the guide-path of which is the averaged equilibrium orbit l8.

FIG. 4 illustrates an alternate lay-out of the betatron electromagnet pole pieces wherein, as distinct from the embodiment illustrated in FIGS. 1 and 2, the pole pieces 3 and 4 are so arranged that the ribs 1, belonging to the pole piece 3, are turned in their plane by the angle a relative to the ribs 1 of the pole piece 4.

The embodiment of the pole-pieces illustrated in FIGS. 1 to 4 is possible because the sectional area of the steel pole-piece steel, ensuring the passage of a magnetic flux necessary to hold the electrons being accelerated in the equilibrium orbit l8, proves to be much smaller than the section of the air-gap in the median plane 5.

It is therefore possible to change the pole-piece configuration by removing part of the steel and to make it instead in the shape of at least three ribs 1 protruding from the central part 2.

In so doing the number, dimensions and shape of the ribs 1 are so selected as to ensure the passage of a magnetic flux necessary to hold the electrons being accelerated in the equilibrium orbit 18.

The rib width for the present pole-piece design is determined with some approximation from this formula where H is the average magnetic-field intensity in the radius r,, of the averaged equilibrium orbit;

H is the magnetic-field intensity within the center insert circle of the radius r N is the number of ribs on a pole.

By varying the profile of the surfaces of the ribs 1, facing the operating zone, it is possible to obtain under the rib a radially decreasing, increasing or nonvarying magnetic field.

Within the space between the ribs 1 where the magnetic field is set up by the leakage fluxes from the side surfaces 14 of the ribs 1, there is always a decreasing magnetic field for the distance between the side surfaces 14 increases radially. In all cases the surfaces 15 of the ribs 1, facing the operating'zone 16', are so selected that the azimuth-averaged field gradient 11 should remain within 0 i l which ensures vertical and radial focusing, as in known betatrons.

Besides, such a field provides an additional vertical focusing force by varying the field intensity in azimuth, as, for instance, in isochronous cyclotrons.

The focusing properties of the controlling field are improved still further by using the pole-piece design shown in FIG. 3.

To improve the focusing properties of the field, the surfaces 15 of the ribs 1, facing the operating zone 16, are made concave so that the field gradient n under the ribs 1 has different signs on different sides of the averaged equilibrium orbit 18.

When the pole-pieces are so designed, the electrons are focused by selecting an equal azimuth-averaged field gradient 5 (e.g., 0.5 0.7) within the annular sector of the operating zone 16 between the control part 2 and the averaged equilibrium orbit 18. Within this sector the distance between the side surfaces 14 of the ribs 1 is small, the field variation depth is low, and its effect on the focusing force magnitude is small.

Beyond the averaged radius of the equlibrium orbit 18 the distance between the side surfaces 14 of the ribs 1 is great, the field variation depth increases, the value of the azimuth-averaged field gradient fimay approach 1 which decreases the focusing forces radially.

Varying the sign of the field gradient n under the ribs 1 at points close to the averaged radius of the equilibrium orbit 18, decreases the value of the azimuthaveraged field gradient fiwhich increases the focusing forces radially and expands the useful aperture of the vacuum chamber.

In addition, the present invention provides for other possible methods of correcting the controlling field, as, for instance, by varying the rib width through appropriate changes in the profile of the side surface 14 of the ribs 1.

The embodiment of the electromagnet, illustrated in FIG. 4, wherein only pole-pieces are shown, makes it possible to obtain an additional increase in the focusing forces by converting the equilibrium orbit 18 into a spatial spiral. This is due to the fact that the field being formed has all its components H H,, H 9 in the median plane 5. Such a field possesses a vertical symmetry. The planes of symmetry pass through the axes 20 of the ribs 1. By varying the angle of rotation a of the ribs 1 it is possible to control the field components. It is advisable to select an angle of rotation a equal to 'rr/N (N is the number of ribs on one of the poles) which provides a favorable periodicity of the field being formed.

In addition, the presence of the azimuth field component H 6 may improve the process of electron injection and capture for acceleration.

The ribs 1 may be made spiral which yields an additional increase in the vertical focusing forces.

What is claimed is:

1. A betatron electromagnet comprising, in combination, a magnetic circuit with pole-pieces, forming an operating zone, each of said pole-pieces having a solid central part with central inserts for increasing the density of the magnetic flux and providing a betatron ratio of substantially 2: l each of said pole-pieces having segments of substantially large height only; a betatron vacuum chamber mounted in said operating zone; at least three ribs protruding from said central part and not exceeding the limits of said operating zone; and a field winding with a field varying over time.

2. A betatron electromagnet as defined in claim 1, wherein said ribs of one of the pole-pieces are located opposite the ribs of the other pole-piece.

3. A betatron electromagnet as defined in claim 2, wherein the surfaces of said ribs facing the operating zone are made concave so that the maximum-distance points of said surfaces relative to the betatron median plane are located near the cylindrical surface, the guide path of which is an averaged equilibrium orbit.

4. A betatron electromagnet as defined in claim 1, wherein the ribs of one of said pole-pieces are turned in their plane relative to the ribs of the other polepiece.

5. A betatron electromagnet as claimed in claim 1, wherein the inner pole gap formed by the internal surfaces of said ribs increases in the pole area from the radius of the central core to the radius of the equilibrium orbit and decreases in the pole area beyond the radius of the equilibrium orbit.

6. A betatron electromagnet as claimed in claim 1, wherein the ribs of one pole-piece are located opposite the gap between the ribs of the other pole-piece. 

1. A betatron electromagnet comprising, in combination, a magnetic circuit with pole-pieces, forming an operating zone, each of said pole-pieces having a solid central part with central inserts for increasing the density of the magnetic flux and providing a betatron ratio of substantially 2:1, each of said pole-pieces having segments of substantially large height only; a betatron vacuum chamber mounted in said operating zone; at least three ribs protruding from said central part and not exceeding the limits of said operating zone; and a field winding with a field varying over time.
 2. A betatron electromagnet as defined in claim 1, wherein said ribs of one of the pole-pieces are located opposite the ribs of the other pole-piece.
 3. A betatron electromagnet as defined in claim 2, wherein the surfaces of said ribs facing the operating zone are made concave so that the maximum-distance points of said surfaces relative to the betatron median plane are located near the cylindrical surface, the guide path of which is an averaged equilibrium orbit.
 4. A betatron electromagnet as defined in claim 1, wherein the ribs of one of said pole-pieces are turned in their plane relative to the ribs of the other pole-piece.
 5. A betatron electromagnet as claimed in claim 1, wherein the inner pole gap formed by the internal surfaces of said ribs increases in the pole area from the radius of the central core to the radius of the equilibrium orbit and decreases in the pole area beyond the radius of the equilibrium orbit.
 6. A betatron electromagnet as claimed in claim 1, wherein the ribs of one pole-piece are located opposite the gap between the ribs of the other pole-piece. 